The present invention relates to a method for optimizing the bulk density and throughput characteristics of coal which is used to form coke, and, in particular, to a method which results in minimizing, and even eliminating, the use of fuel oil previously used for that purpose, and to compositions of matter to be used for those purposes.
Coke is a very important element in the manufacture of steel, and is used in very large quantities in the steel making process. It is formed by treating coal in specially designed ovens in order to produce in the coke a very high percentage of fixed carbon. Coke is the solid residue which remains when certain types of bituminous coals are heated to a high temperature out of contact with air until practically all of the volatile matter has been driven off. The residue consists principally of carbon, with minor proportions of hydrogen, nitrogen, sulphur and oxygen (which together constitute the so-called fixed carbon), plus the mineral matter present in the original coal, which has undergone alteration during the coking process. The process of heating bituminous coal in this manner is referred to as carbonization or coking. The properties of coke depend upon the type of coal, or coal mixture, from which it is made and the process and temperature used in its manufacture.
Coke is essentially a partially graphitized and cellular form of carbon, with a specific gravity generally about mid-way between the specific gravities of coal and graphite. It has a cellular structure and high porosity, and hence has a lower bulk density than coal. It is a combination of high graphitization and porosity that gives coke its chief value in the smelting of iron, where a fuel is required which will burn rapidly in the lower regions of the blast furnace, furnishing a high temperature for the melting of the iron and slag. The coke must also have a high mechanical strength in order to withstand rough treatment and to support a bed of molten iron in a blast furnace.
In order to produce coke of optimum characteristics, the bulk density of the coal charged to the coke oven is of critical importance. Generally speaking, the higher the bulk density of coal, the better the coke that is produced therefrom. If the bulk density is too low, the quality of the coke will be poor due to over-firing and it will not possess sufficient strength for subsequent operations of the steel-making process. On the other hand, if the bulk density of the coal charged to the coke oven is too high, an excessive expansion of the charge in the coking oven may damage that oven.
There are, in general, three factors pointing toward use of coal of higher bulk density. In the first place, increase in coal bulk density increases the thermal conductivity of the oven charge, and this results in greater coking rates and a more uniform distribution of heat. In the second place, the specific gravity of coke varies directly with coal bulk density. An increase of 1 lb/ft.sup.3 in coal bulk density leads to approximately a 1% increase in the specific gravity of coke. Since the specific gravity is a measure of the degree of carbonization, a higher specific gravity means that the coke contains a higher percentage of fixed carbon and that the coking process has been accomplished more completely. In the third place, coke stability and hardness indices vary directly with coal bulk density. An increase in bulk density of 1 lb/ft.sup.3 increases each of those indices by about 0.7. Since coke is used to support a bed of molten metal in a blast furnace, it is desirable to use coke possessing the greatest coal bulk densities lead to coke which is more resistant to shattering upon impact, making it less likely for size degradation to occur during transport and handling.
On the other hand, when coal is coked, it is heated to a temperature at which it fluidizes. As the temperature is raised further, the fluid bed expands as volatile matter is driven off. Once essentially all of the volatile matter has been driven off, the fluid mass solidifies and contracts slightly to form the coke. In some cases, certain coals with excessively high bulk densities may give rise to excessive expansion pressures during coking, and this may damage the oven or its refractory lining. So-called "stickers" may also be formed on the oven walls if the coal bulk density is too high. Consequently, there is an upper limit to bulk density which is determined by coal type and oven construction.
Raw coking coal blends rarely possess the requisite bulk density primarily due to the presence on the coal of surface moisture. Surface moisture decreases the bulk density of formerly dry coking coal. In order to bring the bulk density of coking coal up to desired value, a widely used procedure is to apply fuel oil to the coal. That fuel oil increases bulk density and, to varying degrees, compensates for the effect of surface moisture.
The use of fuel oil for this purpose, however, suffers from three principal disadvantages. In the first place, not only have fuel oil prices risen dramatically in recent years, thereby increasing costs to undesirable levels, but for various economic and political reasons it is essential that the use of fuel oil be minimized. Eighty to ninety million tons of coal are coked annually by steel companies. Currently, steel companies use No. 2 fuel oil at rates of 2.0-20 pints/ton to adjust the bulk density of coking coal charged to the coke ovens. When the coal has fifteen percent (15%) surface moisture, about two gallons of fuel oil are used per ton of coal. At present, the oil costs something over fifty cents per gallon. Thus, for 15% surface moisture coal, the cost to the steel company is about $1.00/ton. 15% surface moisture is a not too frequent ocurrence, but a figure of 11/2 gallons of fuel oil per ton of coal may be a conservative average figure. It is therefore a reasonable estimate that the steel companies use about 150,000,000 gallons of fuel oil per year, at a cost of $75,000,000. Any process which minimizes or eliminates such a use of fuel oil is economically attractive to the steel companies and essential to the country.
In the second place, excessive amounts of fuel oil have been required on wet coal because the wettability of the coal surfaces by oil decreases as the amount of surface moisture increases. Hence, for those coals most in need of bulk density increase, i.e., those coals which have the greatest amount of surface moisture, the amounts of fuel oil that must be used are quite high.
In the third place, if coal has been oxidized, it does not respond as satisfactorily to the fuel oil treatment. Some of the coal produced from strip mining and included in coking coal blends has been exposed to the weathering action of the environment over thousands of years, which causes such oxidation.
Only bituminous coals produce cokes of suitable properties, but not all bituminous coals will do so. Since it is difficult to find a single coal having all of the requisite properties, it is general practice to blend two or more coals into a mix which will perform satisfactorily in the oven and produce a quality coke. Because of the vast quantities of coking coal that are required, coal is piled and stored at coking plants until it is needed. These piles of coal are subjected to the weather and, particularly, to rain, to a greater or lesser degree, and hence it is virtually inevitable that the coal particles will have a relatively high amount of surface moisture thereon, sometimes as much as fifteen percent by weight, and, as has been explained, the existence of such surface moisture not only generally decreases the bulk density the coal but also increases in a greater than linear relationship the amount of fuel oil that must be used to counteract the deleterious effect of the surface moisture.
There is another characteristic of coking coal which is an important industrial consideration, and that is its ability to be moved through equipment, a characteristic often termed "throughput". The better the throughput characteristic of a given mass of coking coal, the more readily will it move through the appropriate equipment, and hence, the less energy will be required to move it therethrough. As the surface moisture of the coal increases, throughput decreases, a not unexpected result--dry coal flows freely, but wet coal does not. When fuel oil is added to coking coal with a high moisture content, although the coking characteristics of that coal are improved, the addition of fuel oil generally aggravates the lessened throughput characteristic produced by the surface moisture. In other words, from a throughput point of view, the addition of fuel oil to coking coal with high surface moisture appears to make a bad situation worse. Hence, any procedure which will improve bulk density of moist coking coal without also decreasing its throughput capacity would be highly desirable.
I have discovered that when suitable combinations of surfactants and other specific substances are added to coking coal, the amounts of fuel oil required to produce coals of requisite bulk density are greatly decreased, and, if certain solid lubricating substances are used, the fuel oil may be dispensed with completely. Moreover, this method results in a marked improvement in the throughput characteristic for the coking coal.
Thus, the need for fuel conservation is satisfied, the throughput characteristic of the coking coal is improved, thereby further reducing energy consumption, and the entire process is less expensive than the prior art process, all without any sacrifice in the operative characteristics of the coal insofar as the production of good coke is concerned. To achieve energy conservation without any sacrifice in overall process efficiency, and to do that with an actual saving of money, is no mean task.
It is a prime object of the present invention to provide a method for enhancing the bulk density of coking coal, and particularly, such coal having surface moisture thereon, which minimizes, and hopefully eliminates, the need for using scarce, expensive and strategically valuable fuel oil for that purpose.
It is a further object of the present invention to provide such a procedure which will also improve the throughput characteristics of the coal.
It is yet another object of the present invention to provide such a procedure which will be no more costly, and hopefully less costly, than the prior art procedures used to accomplish the same result.
It is an additional object of the present invention to provide compositions of material which can be added to coking coals to improve their bulk density and throughput characteristics.
To those ends, and in accordance with the present invention, the particles of the coal in question, either when they are in a pile or, preferably, while they are being transported to the place where they are to be piled or stored, are treated with a material which includes a surfactant having a chain of ten or more carbon atoms and having the characteristic of increasing the spreading coefficient of a second component of the treatment material. In some instances, that second component is fuel oil, used in lesser amount than in the prior art, in which case the surfactant should be oil-soluble and should have the characteristic of increasing the spreading coefficient of the fuel oil. In other instances, fuel oil can be eliminated entirely and substituted by water as the second component, in which case the surfactant should be water-soluble and should have the characteristic of increasing the spreading coefficient of water. When the second component is fuel oil, an alcohol is preferably included in the treatment material. When the second component is water, an inorganic lubricating substance, such as fumed silica, formed of very small substantially rounded, substantially solid particles, is employed. Sufficient of this treatment material is applied to the coal to produce the desired bulk density and to enhance the throughput characteristic of the coal.
To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to a method of improving the bulk density and throughput characteristics of coking coal, and to a composition of material to be used to that end, as defined in the appended claims, and as described in this specification.
There are many variables involved in evaluating the suitability of bituminous coal for coking purposes. One coal varies from another in physical characteristics and chemical composition. A given type of coal may vary in its coking effectiveness, depending upon the length of time that it has been permitted to stand in the presence of oxygen, the degree to which it has been exposed to moisture and rain, etc. Moreover, most coals actually used for coking represent a blend of different types of coal. Accordingly, it is very difficult to generalize as to the specific compositions and proportions of material appropriate to produce a given bulk density or throughput characteristic for coking coal under a given set of conditions. Even with coals of substantially the same surface moisture content, specific treatment materials may differ and specific materials proportions may differ. Accordingly, for any given instance, some experimentation may be required to determine, from the categories of materials here set forth, which particular material or combination of materials, and which proportions of those materials, will give optimum results.
The main operative constituent for the treatment material here disclosed is a surfactant. The term "surfactant" is here used to mean a substance having the property of lowering surface tension and increasing the spreading coefficient of the second component of the treatment material, which may be either fuel oil or water. If the second component is fuel oil, the surfactant should be oil-soluble, while if the second component is water, the surfactant should be water-soluble. The oil-soluble surfactant increases the spreading coefficient of the fuel oil, and the water-soluble surfactant increases the spreading coefficient of the water.
The treatment material of the present invention, as here specifically disclosed, preferably includes a third component. When some fuel oil is employed in the treatment material along with an oil-soluble surfactant, the third component is an alcohol, and preferably such an alcohol selected from the group consisting of those having from 1-8 carbon atoms in their chain. When the treatment material comprises water and a water-soluble surfactant, the third component is an inorganic lubricating substance formed of substantially rounded, substantially solid particles, the bulk of which pass a screen of about 325 mesh, fumed silica being such a substance.